Water-Vapour

The amount of moisture in the air is an exceedingly variable factor, and is usually expressed in terms of relative humidity, that is the ratio, expressed as a per cent., of the amount actually present to that wThich the air could hold if saturated at the same temperature. The absolute humidity, on the other hand, is the weight, in grams, of moisture contained in 1 c.cm. of air.

In the following table are given the numbers of grams of water-vapour contained in 1 cubic metre, saturated at various temperatures, under a normal pressure of 76 cms.

It is seldom that the atmosphere is actually saturated with moisture, and the temperature to which it must be cooled, on any given occasion, in order that its contained moisture may effect its saturation, is termed the dew-point. This is an important point in meteorological studies, for our sensations as to the dryness or moistness of the air are influenced more by the relative than by the absolute humidity. Thus, for example, on a hot summer's day the air may feel very dry and yet contain more water-vapour than would be required for saturation, on a cold winter's morning, under which latter conditions we should experience an acute sense of dampness.

Temperature, ° C.	Magnus	Regnault
-20	1.046	1.058
-10	2.317	2.299
0	4.788	4.868
5	6.725	6.789
10	9.310	9.356
15	12.716	12.738
20	17.152	17.147
30	30.131	30.079
40	50.735	50.677

The water-vapour of the atmosphere is an important factor in preserving equability of temperature, inasmuch as it absorbs a large portion of the heat radiated from the earth. Water-vapour has a density of 9 (H = l), whilst that of dry air is 14.4. In consequence of this the humidity of the air affects its pressure and is thus an important factor in connection with fluctuations in the height of the barometer. The presence of water-vapour in air exerts a pronounced retarding action on the rate of aeration of natural waters. For this reason metals corrode much more rapidly when submerged in water exposed to a dry atmosphere than when the air above is humid.

The amount of Atmospheric Moisture may be estimated gravimetrically by aspirating air through U-tubes containing some desiccating agent such as sulphuric acid or, better, phosphorus pentoxide, and noting the increase in weight. Calcium chloride may be used for approximate estimations, but for accurate work it is not efficient.

A more rapid and convenient method consists in a determination of the dew-point by means of a hygrometer. One commonly used is that devised by Regnault, and consists of a glass tube, the lower end of which is encased in a silver thimble. The tube contains ether, into which dips the bulb cff a sensitive thermometer. Air is aspirated through the ether causing evaporation and cooling. The temperature is noted at which a film of moisture collects on the thimble, and again when it disappears after stopping the aspiration. The mean of these two data is taken as the dew-point.